With diversification of information in recent years, display devices in information field including solid state imaging devices need “beauty, light weight, thinness, and excellence”. In addition, active developments are progressing for low power consumption and fast response. Researches and developments in high precision full color display devices, in particular, are being made widely.
As is well known, in the field of organic electroluminescence (hereinafter also called as organic EL), C. W. Tang et al reported (in Appl. Phys. Lett. 51, 913 (1987)) that high luminance of more than 1,000 cd/m2 was obtained at an applied voltage of 10 V in a device having a layered structure of thin films of organic molecules. Since the report, researches have been promoted directing to practical application of organic EL devices. Similar devices using organic polymer materials are being actively developed as well.
Organic EL devices are superior to liquid crystal display devices in dependence on angle of visibility and response speed. Further, high luminance and high emission efficiency can be expected owing to high current density at low voltage in comparison with inorganic EL devices and LEDs.
Distinctive features of a display device of organic EL are: (i) high luminance and high contrast, (ii) low voltage drive and high emission efficiency, (iii) high resolution, (iv) wide angle of visibility, (v) fast response, (vi) possibility of microstructure and color display, and (vii) light weight and thinness. Because of these features, organic EL devices are expected to apply to flat panel display to perform “beauty, light weight, thinness, and excellence”.
Pioneer Corporation already commercialized green color monochromatic organic EL display for vehicle mounting in November 1997. To meet diversifying social needs, practical implementation of organic EL multicolor display devices is eagerly expected that exhibit long-term stability, fast response, multicolor display, and capability of high precision full-color display.
Multicolor or full-color display of an organic EL color display device can be performed by a method in which emission elements for three primary colors, red, green, and blue, are separately arranged in a matrix form and each element emits light. (Refer to Japanese Unexamined Patent Application Publication Nos. S57-157487, S58-147989, and H3-214593.) To obtain multicolor by using organic light emitting elements, three types of light emitting materials for R, G, and B must be arranged in a matrix form with high precision. Therefore, the method involves technical difficulty and does not allow production with low cost. Further, difference in lifetimes of three light emission materials reveals a drawback that the chromaticity shifts with time.
Another method is known in which a backlight emitting white light and color filters are used and three primary color light transmits the filters. (Refer to Japanese Unexamined Patent Application Publication Nos. H1-315988, H2-273496, and H3-194895.) However, an organic light emitting element has not been found that emits bright white light required to obtain bright RGB and that exhibits long life.
Still another method is also known in which light from a light emitting element is absorbed in fluorescent elements separately arranged in a plane and each fluorescent element emits fluorescent light in each different color. (Refer to Japanese Unexamined Patent Application Publication No. H3-152897.) The method disclosed in this reference, in which a light emitting element is used to obtain multiple colors through fluorescent elements, is applied to CRT and plasma display.
In recent years, a color conversion system has been disclosed in which filters of fluorescent material are used that absorb light from an organic light emitting element and emit fluorescent light in a visible light range. (Refer to Japanese Unexamined Patent Application Publication Nos. H3-152897 and H5-258860.) The light to be emitted from the organic light emitting element is not limited to white light in this system. Therefore, brighter organic light emitting element can be used for a light source. A color conversion system using an organic light emitting element that emits blue light converts the emitted blue light to green light and red light through fluorescent materials by wave length conversion. (Refer to Japanese Unexamined Patent Application Publication Nos. H3-152897, H8-286033, and H9-208944.) By patterning a fluorescent light conversion film containing fluorescent dyes with high precision, a full-color light emitting type display can be constructed using relatively low energy radiation such as near ultraviolet or visible light from the light emitting element.
Methods for pattering a color conversion filter are disclosed as follows.    (a) As in the case of an inorganic fluorescent element, fluorescent dye is dispersed in liquid of resist (photo-reactive polymer). After forming a film by spin-coating or another method, pattering is conducted by photolithography. (Refer to Japanese Unexamined Patent Application Publication Nos. H5-198921 and H5-258860.)    (b) Fluorescent dye or pigment is dispersed in basic binder. A film of the above material is patterned by etching in an acidic aqueous solution. (Refer to Japanese Unexamined Patent Application Publication No. H9-208944.)
The color conversion system that is regarded promising for obtaining a multicolor display device, however, has a problem that the fluorescent filter for converting to a fluorescent light with the target wavelength is extremely weak against light with a special wavelength, moisture, heat, and organic solvent. These factors readily decompose the material of the fluorescent filter and causes loss of functions of the filter. Consequently, constraints are posed on the manufacturing process of multicolor organic EL devices.
In a conventional process of manufacturing a multicolor organic EL device of the color conversion system, the process (film deposition, photolithography) of forming a transparent ITO electrode, which is an anode of an organic light emitting element, is liable to deteriorate the conversion characteristics (conversion efficiency, color purity) of the color conversion filter by heat generated in the process. Therefore, it is difficult to form an organic light emitting element directly on a protective layer.
To solve this problem, a so-called inversion structure has been proposed. In the structure, first cathode films are deposited on a transparent substrate. Then, an organic layer, anodes, a protective layer, and color conversion filters are sequentially formed. The structure, however, involves a problem that the organic light emitting layer deteriorates in a wet process for forming the color conversion filters. The organic light emitting layer is sensitively affected by the environment, in particular, moisture. Moisture promotes crystallization of the organic layer. Moisture also penetrates between the electrode and the organic light emitting layer and causes cleavage between them, thereby generating dark spots, in which light emission fails on voltage application. In the inversion structure, a cathode is formed on a glass substrate of the device. Since the cathode and the anode must be orthogonal, a process of patterning the cathode (photolithography process) is necessary. Cathode material of a metal or an alloy with low work-function such as Al—Li is oxidized and electron injection performance deteriorates, causing degradation of light emission efficiency of the device.
To solve this problem, a method of manufacturing an organic EL device has been proposed (Japanese Unexamined Patent Application Publication No. 2001-93664). The method comprises a step of forming an organic light emitting element by laminating a cathode, an organic light emitting layer, and a transparent electrode on a first substrate, a step of forming a color conversion filter on a transparent second substrate, the filter receiving electroluminescence from the organic light emitting layer and emitting fluorescent light, and a step of arranging the first and the second substrates opposite to each other in such a way that the organic light emitting element and the color conversion filter are sandwiched by the substrates.
Important performance for practical application to color display is long term stability of light emitting characteristics as well as precise color display performance (Refer to Kinou Zairyou: Vol. 18, No. 2, p 36-(published in 1998)). Nevertheless, organic EL devices have a drawback that light emitting performances including current—luminance characteristic significantly degrade after certain period of operation.
Principal cause of the deterioration of light emission performance is growth of dark spots; the dark spot is a defect spot of light emission. The growth of dark spot proceeds along with progress of oxidation in the operating period and the storing period, and the dark spots expand towards whole of the light emission surface.
The dark spot is considered to be originated from oxidation or aggregation of materials of the laminated layers of the light emitting element caused by oxygen or moisture contained in the element. The growth of dark spots proceeds in storage as well as in operation. The growth of dark spots specifically is:    (a) accelerated by oxygen or moisture around the element;    (b) affected by oxygen or moisture adsorbed on the laminated organic films; and    (c) affected by moisture that is adsorbed on the parts for manufacturing the device or moisture that invades in the manufacturing process. To attain the long term stability of light emission performance, the growth of dark spots must be suppressed.
A color conversion filter, which is a mixture of a resin and color conversion dyes contained in the resin as described previously, does not allow drying at a temperature over 200° C. because of thermal stability of the mixed dyes. Consequently, a color conversion filter is apt to be formed in a condition retaining moisture that is contained in the coating liquid or that has invaded in the patterning process. The factor to promote the dark spot growth is the moisture contained in the color conversion filter or the moisture arrived into the sealed off region through a protective layer in the period of storage or operation.
The following describes, referring to a drawing, a principal factor of the dark spot growth that has been found based on the study by the inventors.
FIG. 4 is a sectional view of a conventional organic EL multicolor display device using a color conversion filter. The symbols in FIG. 4 represent;
1: a transparent substrate, 2: a red color conversion filter, 3: a green color conversion filter, 4: a blue color conversion filter, 5: a flattening layer, 6: a protective layer, 7: a transparent lower electrode (anode), 8: a hole injection layer, 9: a hole transport layer, 10: an organic light emitting layer, 11: an electron injection layer, 12: an upper electrode (cathode), 13: a desiccant, and 14: sealing glass.
FIG. 4 shows a four-layer structure of organic thin film layers comprising a hole injection layer 8, a hole transport layer 9, an organic light emitting layer 10, and an electron injection layer 11. Moisture or solvent penetrates into the organic thin film layers sandwiched between the transparent lower electrode 7 and the upper electrode 12. With the diffusion of the moisture or solvent, aggregation or crystallization occurs at the operating temperature in the molecules composing the organic thin film layers. As a result, adhesiveness with the lower electrode or with the upper electrode lowers, or the distance between the lower electrode and the upper electrode increases, which is the reason for the dark spots.
Accordingly, removal of the moisture can be a means to suppress the dark spot growth. Desiccating methods have been proposed including a method that provides a desiccant of phosphorus pentoxide in the internal space of the device and seals off the space (Refer to Japanese Unexamined Patent Application Publication No. H3-261091), and a method that constructs a lamination structure comprising a protective layer and a sealing layer containing phosphorus pentoxide. (Refer to Japanese Unexamined Patent Application Publication No. H7-169567.) In these methods, however, the desiccant of phosphorus pentoxide absorbs the moisture to change into phosphoric acid and adversely affects the organic lamination. Other methods have been further proposed including a method in which the space over the laminate and the space within the sealed container are filled with inactive liquid containing desiccant agent (Refer to Japanese Unexamined Patent Application Publication Nos. H5-41281 and H9-35868), and a method that uses a pressure-sensitive adhesive (Refer to U.S. Pat. No. 5,304,419). But, the methods have not reached a satisfactory solution.
FIG. 4 is, as described above, a schematic sectional view of the whole structure of a conventional organic light emitting device using a color conversion filter. The following describes the structure of FIG. 4 in more detail and clarify the problem to be solved by the present invention.
FIG. 4 shows a part corresponding to one pixel of an organic light emitting apparatus comprising multiple pigments for use in multicolor or full-color display. Color conversion filters 2, 3, and 4 containing red, green and blue dyes or pigments are formed on a transparent substrate 1. Then, a flattening layer 5 and a protective layer 6 are formed. The organic light emitting device further comprises transparent anodes 7 (transparent lower electrodes) of ITO or the like formed on the protective layer 6 by patterning, a hole injection layer 8 covering the anodes 7, a hole transport layer 9 formed on the hole injection layer 8, an organic light emitting layer 10 formed on the hole transport layer 9, an electron injection layer 11 formed on the organic light emitting layer 10, and a cathode 12 (an upper electrode) of a metal or an alloy formed on the electron injection layer 11. Each of the groups of the transparent electrodes 7 and the upper electrodes 12 are formed as a line pattern with a predetermined gap between the lines. The two line patterns are arranged orthogonal with each other. Resulted parts of an organic light emitting device is put into a glove box filled with a dry nitrogen atmosphere (oxygen content and moisture content are below 10 ppm) and are adhered to a sealing glass 14 provided with a desiccant 13 using a UV-curing adhesive.
In a case of a structure of an organic light emitting device that is different from the structure of FIG. 4 and comprises a first substrate having the organic light emitting layer and another substrate having a color conversion filter layer, the two substrates being stuck together, the conventional desiccating member as shown in FIG. 4 cannot be provided because of structural or spatial restriction since the conventional structure needs to occupy certain space for the desiccating member. As a result, such an organic light emitting device lacks long term reliability.
Therefore, a problem to be solved by the invention is a problem involved in the above described “an organic EL light emitting device comprising a first substrate having an organic light emitting layer and another substrate having a color conversion filter layer, the two substrates being stuck together”, and an object of the invention is to produce and provide such an organic EL multicolor display device of a color conversion system that maintains stable light emitting performance for a long period and exhibits excellent angle of visibility.